1. Field of the Invention
This invention relates to improvements to a process for the formation of a hybrid chemical conversion coating on ferrous metal substrates, consisting of an iron/oxygen rich intermediate coating and a top layer of magnetite. This invention also relates to ferrous metal substrates coated according to the presently disclosed improved process. This invention further includes improvements to the oxidation solution used in oxidizing the iron/oxygen rich intermediate coating to the final magnetite containing top layer. This invention also includes improvements to a seven-step procedure for preparing a ferrous metal substrate with a magnetite containing coating.
2. Description of the Related Art
Prior, commonly-assigned U.S. Pat. No. 6,309,476 and Ser. No. 09/710,187 describe a method for forming a chemical conversion coating on ferrous metal substrates, the chemical solutions used in the coating and the articles coated thereby. U.S. Pat. No. 6,309,476 and Ser. No. 09/710,187 will be referred to herein as the Ravenscroft disclosures. Those inventions modified and combined features of two existing, but previously unrelated, coating technologies, to form a hybrid conversion coating. The Ravenscroft disclosures described molecular iron/oxygen-enriche intermediate coatings, such as a dicarboyxlate or phosphate, applied to a ferrous substrate by a first oxidation. The intermediate coating pre-conditioned the substrate to form a surface rich in molecular iron and oxygen in a form easily accessible for further reaction. The first oxidation reaction of the Ravenscroft disclosures preceded a coloring process (second oxidation) using a heated oxidizing solution that reacted with the iron and oxygen enriched intermediate coating to form magnetite. The result of the process of the Ravenscroft disclosures was the formation of a brown or black finish under milder and safer conditions than had previously been seen with conventional caustic blackening procedures, due to the chemical reaction between the intermediate coating and the second oxidation solution. When sealed with an appropriate rust preventive topcoat, the result of the Ravenscroft procedures was an ultra-thin, attractive and protective finish applied through immersion techniques. The finish was a final protective coating on a fabricated metal article and afforded a degree of lubricity to aid assembly, break-in of sliding surfaces, provided anti-galling protection, and provided an adherent base for paint finishes.
The established art of coloring ferrous metals has revolved principally around methods for producing black coatings. Since the 1950's, the most commonly used commercial method for blackening ferrous metals has been the caustic black oxidizing process. This disclosure will examine this method, along with the ferrous oxalate conversion coating on ferrous metal substrate and the iron phosphatizing process.
Caustic black oxidizing: This process uses sodium hydroxide, sodium nitrate and sodium nitrite as oxidizing agents, operating at about pH 14, at temperatures of about 285-305° F. A black coating forms during exposures of about 10-30 minutes. This process forms a magnetite (Fe3O4) deposit, approximately 1 micron thick, by reacting with the metallic iron substrate in situ. Although the process produces high quality black finishes when operated properly, it has the disadvantage of requiring high temperatures and highly concentrated solutions (700-1000 grams per liter) to carry out the reaction.
During the course of operation, this reaction consumes oxidizing salts and the solution boils off significant quantities of water. Adding these materials back to the solution maintains proper operating conditions. However, adding sodium hydroxide to water, being a highly exothermic reaction, is quite hazardous because the operating solution is already boiling. Likewise, adding make-up water to a solution that is already at 285-305° F. causes the water to boil instantly if not added very slowly and carefully. Consequently, the operation of the process poses severe safety hazards for personnel, due to the dangers involved in normal system operation and maintenance. These hazardous conditions may be difficult to justify in the manufacturing environments of modern industry. In addition, normal operating conditions typically entail heavy sludge formation in the process tank, difficulty in disposal of the spent solutions (due to extremely high concentrations), and variable quality on certain metals, including tool steel alloys, sintered iron articles or other porous substrates. Without the use of highly skilled operators, this process may result in poor quality finishes. It is common to see undesirable red/brown finishes on certain alloys or salt leaching on porous substrates. As a result, the process largely requires the use of professional metal finishers who possess specialized knowledge and experience in dealing with hazardous materials.
Ferrous oxalate conversion coating: The development of this coating originally provided resulted in a metal forming lubricant and anti-galling coating for mating parts. Application of the finish is generally at about ambient temperatures. The finish is about one micron thick and opaque gray in color. When sealed with a rust preventive topcoat, the oxalate offers some degree of corrosion protection. Used more commonly in the 1950's, the oxalate process is rarely used today, having given way to the several phosphate processes on the market, which offer more beneficial properties in terms of lubrication and/or paint adhesion.
Iron phosphate conversion coating: These coatings are widely used in the metal finishing industry as pretreatments to enhance paint adhesion and corrosion resistance on ferrous metal substrates. With a coating thickness of about 1 micron, the amorphous deposit forms at temperatures of about 70-130° F. by a mildly acid solution that may also contain cleaning agents. The iron phosphate process has proven to be a very versatile and effective option in paint lines and other metal finishing process lines.
There have been several patents issued over the years that relate to blackening processes. For purposes of this invention, however, the following prior patent references directly relate to oxalate and phosphate conversion coatings on ferrous metal substrates and to the caustic black oxidizing of ferrous metal substrates:
U.S. Pat. No.DateSubject2,774,696Dec. 18, 1956Oxalate Coatings on Chromium AlloySubstrates2,791,525May 7, 1957Chlorate Accelerated Oxalate Coatingson Ferrous Metals for FormingLubricity and Paint Adhesion2,805,696Sep. 10, 1957Molybdenum Accelerated OxalateCoatings2,835,616May 20, 1958Method of Processing Ferrous Metals toForm Oxalate Coatings2,850,417Sep. 2, 1958m-Nitrobenzene Sulfonate AcceleratedOxalates on Ferrous Metals2,960,420Nov. 15, 1960Composition and Process for BlackOxidizing of Ferrous Metals UsingMercapto-Based Accelerators andnaphthalene based Wetting Agents3,121,033Feb. 11, 1964Oxalates on Stainless Steels3,481,762Dec. 2, 1969Manganous Oxalates Sealed withGraphite and Oil for Forming Lubricity3,632,452Jan. 4, 1972Stannous Accelerated Oxalates onStainless Steels3,649,371Mar. 14, 1972Fluoride Modified Oxalates3,806,375Apr. 23, 1975Hexamine/SO2 Accelerated Oxalates3,879,237Apr. 22, 1975Manganese, Fluoride, SulfideAccelerated Oxalates3,899,367Aug. 12, 1975Composition and Process for BlackOxidizing of Ferrous Metals UsingMolybdic Acids on Tool Steels4,017,335Apr. 12, 1977pH Stabilized Composition and Methodfor Iron Phosphatizing of Ferrous MetalSurfaces5,104,463Apr. 14, 1992Composition and Process for CausticOxidizing of Stainless Steels UsingChromate Accelerators
All but one of these oxalate patents pertain to the formation of a ferrous oxalate conversion coating on ferrous metal substrates using various accelerators. These oxalates are function as coatings to aid in assembly or provide forming lubricity, etc. These coatings serve as deformable or crushable boundary layers at the metal surface, thereby protecting the base metal during contact with another surface.
The caustic black oxidizing patents focus on compositions and processes that oxidize the metallic iron substrate to a magnetite, Fe3O4, as described in U.S. Pat. No. 2,960,420. Actually, when examining the stoichiometry of the Fe3O4, one can see that the iron is not in either a purely ferrous (II) or ferric (III) oxidation state. Perhaps a more precise description of the material is that of a mixed salt, ferrosoferric oxide, or FeO.Fe2O3, which exhibits both ferrous and ferric iron. The conventional caustic oxidizing processes all depend on the ability of the operating solution to oxidize metallic iron to both ferrous (II) and ferric (III) oxidation states to form the mixed oxide FeO.Fe2O3.
The process described in U.S. Pat. No. 4,017,335 is representative of the state of the art, focusing on the well-known primary phosphatizing mechanism. In addition, this same patent illustrates incorporation of a cleaning agent, pH stabilizer into the oxidizing solution to effectively clean lightly soiled ferrous articles, and iron phosphatize them in a single step.